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Python: Elegant and efficient ways to mask a list

I have two equal sized lists, something like:

a=["alpha","beta","kappa","gamma","lambda"]
b=[1,2,None,3,4,5]

What I would like to do is identify and delete the none element in list [b] and then remove the corresponding element in list [a]. Here, for example, I would like to delete none and “kappa”.

I am aware of:

filter(bool,b)

which would remove the None elements from [b], but, how do I go about deleting the corresponding entry in list[a]?

I tried zip, something like (the idea was to pack and unpack):

a=["a","b","c","d","e"]
b=[1,2,None,3,4]
c=zip(a,b)
d=filter(bool,c)

..but this does not work. [d] still has the none elements.

I would appreciate any pythonic way to achieve this.

Answer 1

This can be done neatly with itertools.compress() and a list comprehension:

>>> a=["a", "b", "c", "d", "e"]
>>> b=[1, 2, None, 3, 4]
>>> selectors = [x is not None for x in b]
>>> list(itertools.compress(a, selectors))
['a', 'b', 'd', 'e']
>>> list(itertools.compress(b, selectors))
[1, 2, 3, 4]

This method means you only generate the selectors once (and itertools.compress() should be nice and fast).